The identification, measurement and/or control of physical assets are important aspects of modern business practices. Frequently, assets are misidentified, misplaced or incorrectly dispensed, thereby leading to incorrect inventory and/or receivables.
A common modern method for dealing with asset control is the use of bar codes. These bar codes can be used to both identify a product and support the determination of the time and location of dispensation.
Another increasingly common method for asset control is the use of radio frequency tags (RF tags). These are tags that are attached to assets and that include at least a radio transmitter and identification circuit. The identification circuit continually, periodically, or after an interrogatory is sent from a receiver, sends the identification of the product.
These systems, while excellent for product identification, are not optimized for tracking events that may occur to the products. These events may be movement of the asset, tilting of the asset, acceleration of the asset, changes in temperature of the asset, breakage of the asset (or associated tag), button presses, and the like.
Therefore, there is a present and continuing need for improved asset tags used for the identification, measurement and/or control of physical assets.
Asset tags desirably communicate data describing the events they track to other devices for processing that data. In many situations, it is convenient to use radio-frequency (RF) transmissions to communicate the data. But conventional RF communication techniques fail to address the needs of systems that rely upon asset tags, and conventional RF communication techniques are not well suited to other types of RF communications apparatuses as well.
Most electronic systems benefit from lower cost components. But systems that use asset tags as well as other types of electronic systems have a particularly heightened need for low cost components. The need for a low cost component is heightened when a particular device, such as an asset tag, is used in large numbers by a given system. In this situation, any unnecessary costs are multiplied by the number of the often-used device in the system.
And, many electronic systems, including those that include asset tags, benefit from components of smaller size. When asset tags are associated with products, the asset tags need to be as small as possible so that they do not detract from the packaging and ambiance, so that they do not take up significant space that is better used by the products with which they are associated, and so that they do not interfere with the operation and manipulation of the products, their packaging, or their containers.
Likewise, most electronic systems can benefit from operation with the lowest possible power consumption. But systems that rely upon asset tags and other types of electronic systems have a heightened need for low-power operation. When a device, such as an asset tag, relies upon the use of one or more batteries to provide its electrical power, the selected battery often drives many design parameters for the device.
Greater battery capacity can lessen the pressures for achieving low-power operation. Greater battery capacity can be achieved by using more expensive batteries of a given size, larger batteries of a given battery technology, by using a greater number of batteries, by using rechargeable batteries, and/or by requiring occasional replacement of batteries. But each of these options is undesirable. A more expensive battery, a larger battery, or a larger number of batteries poses a cost problem. Accordingly, these are undesirable solutions when a heightened need exists for low cost. And, larger batteries or a greater number of batteries cause a battery-powered device, such as an asset tag, to be larger than it might be. Again, these are undesirable solutions when a need exists for making an RF apparatus as small as possible.
Rechargeable batteries are also undesirable to the extent that they are more expensive than non-rechargeable batteries. And, expenses and size requirements are further increased by an undesirable need to recharge the batteries and to provide the associated recharging circuits and related paraphernalia.
The use of replaceable batteries is also undesirable in some applications because the ongoing need to purchase replacement batteries increases costs in many electronic applications, such as those that rely upon asset tags. But replaceable batteries and/or rechargeable batteries are undesirable in asset tag and other electronic applications for other reasons as well. RF apparatuses that use rechargeable and/or replaceable batteries will be required to operate on low battery reserves from time to time. This will result in an unreliable operation. And, when the battery reserves are finally exhausted, they impose a nuisance factor on the user who is denied the services that RF apparatus should be providing and is then required to either recharge or replace batteries. In electronic systems that may use several battery-powered devices, such as systems that rely upon asset tags, this nuisance factor is a serious problem.
Accordingly, asset tags and many other electronic devices can benefit from a capability to engage in RF communications, to be as small as possible, to be as inexpensive as possible, and to be powered by one or more batteries that are as small and inexpensive as possible, yet are non-replaceable if at all possible.
Engaging in RF communications on tight cost, power, and space budgets is an extremely challenging task. One of the factors that exerts a substantial influence on this task is the antenna through which RF communications takes place. A loop antenna is a conductive loop which is tuned using a tuning capacitor coupled to the loop to resonate at a desired RF frequency. Conventional loop antennas exhibit many desirable characteristics for these types of applications. For example, they can be formed in a small space. And, they can be configured to exhibit a high quality factor (Q), which allows them to operate at a somewhat greater power efficiency for a given loop size.
But conventional loop antennas fail to achieve the space and efficiency goals that would be beneficial for asset tags or other RF communications devices. One reason for this failure is that as loop antennas get smaller to meet tight space requirements, they then need to be operated at as high a Q as possible to maximize their power efficiency. This makes a loop antenna highly sensitive to tuning. In other words, if the tuning capacitor exhibits a capacitance as little as a couple of percent off of the ideal value which achieves resonance at a desired RF frequency, power efficiency can suffer tremendously. But, RF devices on tight power budgets cannot afford reduced power efficiency.
The sensitivity to tuning of conventional high Q antennas poses another problem. Governmental regulatory agencies, such as the Federal Communications Commission (FCC) in the United States and counterparts in other countries, restrict the amount of power that can be broadcast from an antenna. Manufacturers are required to reduce power output based on a worst likely case manufacturing sample. The sensitivity to tuning of a high Q antenna means that when the antenna cannot be consistently tuned, transmit power will need to be reduced to meet regulations, and the radio range will be reduced from what it might be if antennas could be more consistently tuned. And, the regulations tend to be more strict for high volume, mass market transmission applications. These are the same applications where cost concerns are strongly felt.
Conventional loop antennas in these situations use discrete, manually-tuned, board-mounted tuning capacitors, discrete, high precision, board-mounted tuning capacitors, discrete, highly stable, board-mounted tuning capacitors, and/or discrete, pre-screened, board-mounted tuning capacitors. Discrete board-mounted capacitors are leaded or surface-mount capacitors that are mounted on a printed wiring board. But, manually-tuned and pre-screened tuning capacitors are simply not compatible with mass-market manufacturing techniques where large numbers of devices need to be manufactured on a tight cost budget. And, high precision and/or highly stable capacitors are so expensive that they also are undesirable in applications on a tight cost budget. In such situations, conventional loop antennas couple resistive elements to the loop antenna to reduce the Q to the point where a tuning capacitor that meets budgetary requirements can effectively tune the antenna. But in a battery powered device on a tight power budget, techniques that lead to such power inefficiencies are undesirable.